1. Field of the Invention
The present invention relates to a heat-sensitive recording material. In particular, the present invention relates to a heat-sensitive recording material characterized by containing a tetrahydronaphthalene compound as sensitizer.
2. Description of the Prior Art
Heat-sensitive recording materials consist of a heat-sensitive color-forming layer comprising a dispersion of a sensitizer, binder and other additives in a coupling system comprising a usually colorless or light-colored coupling substance such as a leuco dye and a developer which causes coloring of the coupling substance upon heating, said layer being formed on a support such as paper, synthetic paper or resin film. When a heating element such as a thermal head or hot pen is brought into contact with the recording material in a recording device, the dye is reacted with the developer to develop a color such as black to thereby form a record.
The heat-sensitive recording materials are widely used in instrumental recorders, computers, facsimiles, telex devices, automatic passenger ticket vending machines, etc., since they are superior to other recording materials in that the records can be obtained in a short time, the noise is only slight and they are inexpensive.
As the colorless or light-colored coupling substances, for example, leuco dyes having a lactone, lactam or spiropyran ring are used. As the developers, various acidic substances have been proposed heretofore. Among them, phenolic compounds such as bisphenol A and benzyl p-hydroxybenzoate are frequently used either alone or in combination of them. However, when these phenols are used, increase of the recording speed and density is difficult, color shading is caused or discoloration occurs during the storage disadvantageously.
Attempts have been made to add a sensitizer to a coupling substance and a developer to thereby increase the sensitivity. The sensitizers which have been used include waxes, stearamide, and terphenyl.
However, these sensitizers are practically unsatisfactory, since they have defects that the effects of them cannot be sufficiently exerted unless they are used in a large amount, that a high sensitization of the heat-sensitive recording material required thereof cannot be sufficiently satisfied and that fogging takes place or discoloration occurs during the storage.
Under these circumstances, Japanese Patent Laid-Open No. 64593/1982 proposes the use of .beta.-naphthyl benzoate, Japanese Patent Publication No. 25674/1984 proposes the use of substituted biphenylalkanes such as 1,2-bis(2,4-dimethylphenyl)ethane, Japanese Patent Laid-Open No. 56588/1985 proposes the use of diaryloxyalkanes such as bis(naphthyloxy)alkanes, and Japanese Patent Laid-Open No. 178087/1985 proposes the use of diaryl carbonates such as naphthyl phenyl carbonate.
However, although the coloration sensitibity is improved to some extent by using such a compound, the colored image area is discolored (disappearance of color) or the non-image area gives rise to color (fogging) disadvantageously. Another defect is that the color density of the print produced by using a heat-sensitive recording material comprising such a sensitizer after it has been stored for a long period of time is far lower than that of the print produced by using the same heat-sensitive recording material before storage.